1. Field of the Invention
Embodiments of the present invention relate to polyurethane foams. More particularly, embodiments of the present invention relate to polyurethane foams having viscoelastic properties.
2. Description of the Related Art
Polyurethane foams are used in a wide variety of applications, ranging from cushioning (such as mattresses, pillows and seat cushions) to packaging to thermal insulation and for medical applications. Polyurethanes have the ability to be tailored to particular applications through the selection of the raw materials that are used to form the polymer.
One class of polyurethane foam is known as viscoelastic (VE) or “memory” foam. Viscoelastic foams exhibit a time-delayed and rate-dependent response to an applied stress. They have low resiliency and recover slowly when compressed. These properties are often associated with the glass transition temperature (Tg) of the polyurethane. Viscoelasticity is often manifested when the polymer has a Tg at or near the use temperature, which is room temperature for many applications.
Like most polyurethane foams, VE polyurethane foams are prepared by the reaction of a polyol component with a polyisocyanate, in the presence of a blowing agent. The blowing agent is usually water or a mixture of water and another material. VE formulations are often characterized by the selection of polyol component and the amount of water in the formulation. The predominant polyol used in these formulations has a functionality of about 3 hydroxyl groups/molecule and a molecular weight in the range of 400-1500. This polyol is primarily the principal determinant of the Tg of the polyurethane foam, although other factors such as water levels and isocyanate index also play significant roles.
Compression set is a measure of the permanent deformation of a foam after it has been held compressed under controlled time and temperature conditions. Compression set is generally used to correlate in-service loss of cushion thickness and changes in foam hardness. Lower compression set values are generally desirable for viscoelastic polyurethane foams.
Typically viscoelastic polyurethane foams have low air flow properties, generally less than about 1.0 standard cubic feet per minute (scfm) (0.47 liters/second) under conditions of room temperature (22° C.) and atmospheric pressure (1 atm), therefore promoting sweating when used as comfort foams (for instance, bedding, seating and other cushioning). Low air flow also leads to low heat and moisture transfer out of the foam resulting in (1) increased foam (bed) temperature and (2) moisture level. The consequence of higher temperature is higher resiliency and lowered viscoelastic character. Combined heat and moisture result in accelerated fatigue of the foam. In addition, if foam air flows are sufficiently low, foams can suffer from shrinkage during manufacturing.
It would be desirable to achieve improved compression set values than is generally now achieved while retaining viscoelastic properties of the foam. Furthermore, it would be desirable to have foams with improved compression set values while retaining properties such as air flow.